Relating to magnetic resonance imaging

ABSTRACT

The invention shows a method for obtaining temporally spaced images of tissues, including blood vessels, to reveal blood flow abnormalities within those tissues and vessels.

RELATED APPLICATIONS DATA

This is a continuation of application Ser. No. 08/604,778 (filed Feb.23, 1996, now U.S. Pat. No. 5,833,947), which was a divisional ofapplication Ser. No. 08/306,221 (filed Sep. 14, 1994, now U.S. Pat. No.5,494,655), which was a continuation of application Ser. No. 07/946,373(filed Oct. 30, 1992, abandoned). Ser. No. 07/946,373 was filed pursuantto 35 U.S.C. § 371 as a national stage application deriving from PCTApplication No. PCT/EP91/00443 (filed Mar. 6, 1991, Publication No. WO91/14186), and is a continuation-in-part of application Ser. No.07/490,859 (filed Mar. 9, 1990, now U.S. Pat. No. 5,190,744).

This invention relates to improvements in and relating to magneticresonance imaging, in particular imaging of phenomena associated withblood flow variations and abnormalities.

Magnetic resonance imaging (MRI) has been used successfully to studyblood flow in vivo. Moreover Villringer et al. Magnetic Resonance inMedicine 6:164–174 (1988), Cacheris et al. Society of Magnetic Resonancein Medicine, 7th Annual Meeting, San Francisco, 1988, (SMRM 1988) Worksin Progress, page 149 and Belliveau et al. SMRM 1988, Book of Abstracts,page 222 have proposed the use of certain paramagnetic lanthanidechelates as magnetic susceptibility, that is T₂* shortening, MRIcontrast agents for studies of cerebral blood flow and perfusion.

Unlike many previous imaging procedures, T₂ or T₂*-weighted MRI usingmagnetic susceptibility (MS) contrast agents (hereinafter MS imaging)enabled blood perfusion deficits, e.g. cerebral ischemias, to bevisualized rapidly as the MR signal intensity was reduced in the regionsof normal perfusion due to the effect of the contrast agent, withischemic tissue being revealed by its retention of signal intensity.

Blood perfusion deficits are associated with several serious and oftenlife-threatening conditions. Rapid identification and location of suchdeficits is highly desirable in order that the appropriate correctiveaction, be it therapeutic or surgical, may be taken promptly. Thus inthe case of cerebral ischemia, any delay in post ischemic recirculationand reoxygenation of brain tissue reduces neuronal survivability.

MS imaging therefore represents a major improvement over routine T₂ orT₂*-weighted imaging in the absence of MS contrast agents, since in theroutine procedures ischemias or infarcts only become detectable 2 to 3hours after the event, e.g. a stroke, which gave rise to the perfusiondeficit. However, while determination of the existence and location of aperfusion deficit is important, it is also desirable to be able todetect the degree or severity, and if possible the onset and duration ofblood flow abnormalities or variations, in a quantifiable manner. We nowpropose that this be done using a modified MS imaging procedure.

SUMMARY OF THE INVENTION

Viewed from one aspect the invention provides a method of detectingblood flow abnormality or variation in a human or non-human, especiallymammalian, body, said method comprising administering into the systemicvasculature of a said body a contrast enhancing amount of anintravascular paramagnetic metal, e.g. transition metal or lanthanide,containing magnetic resonance imaging contrast agent, subjecting saidbody to a magnetic resonance imaging procedure capable of generatingfrom magnetic resonance signals from said body a series of temporallyspaced images of at least a part of said body into which said agentpasses, and detecting temporal variations in said signals or imageswhereby to identify regions of abnormal or modified blood flow in saidbody and to indicate the degree of blood flow abnormality ormodification therein.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows an MR image of a cat brain 108 minutes post-occlusion of amiddle cerebral artery.

FIG. 2 shows an MR image of a cat brain 160 minutes post-occlusion of amiddle cerebral artery.

FIG. 3 shows an MR image of a cat brain 320 minutes post-occlusion of amiddle cerebral artery.

FIG. 4 shows a contour map of a cat brain 108 minutes post-occlusion ofa middle cerebral artery, wherein the regions of signal hyperintensityspatially tissue perfusion deficits.

FIG. 5 shows a contour map of a cat brain 160 minutes post-occlusion ofa middle cerebral artery wherein the regions of signal hyperintensityspatially demarcate tissue perfusion deficits.

FIG. 6 shows a contour map of a cat brain 320 minutes post-occlusion ofa middle cerebral artery wherein the regions of signal hyperintensityspatially demarcate tissue perfusion deficits.

FIG. 7 shows an MR image of a cat brain 108 minutes post-occlusion of amiddle cerebral artery with superimposed corresponding contour map ofsignal hyperintensity.

FIG. 8 shows an MR image of a cat brain 160 minutes post-occlusion of amiddle cerebral artery with superimposed corresponding contour map ofsignal hyperintensity.

FIG. 9 shows an MR image of a cat brain 320 minutes post-occlusion of amiddle cerebral artery with superimposed corresponding contour map ofsignal hyperintensity.

FIG. 10 shows a histopathologic brain section.

FIG. 11 shows the histopathologic brain section with superimposedstaining contours.

FIG. 12 shows an echo planar MR image of cat brain prior to occlusion ofa middle cerebral artery.

FIG. 13 shows an echo planar MR image of cat brain during occlusion ofthe middle cerebral artery.

FIG. 14 shows an echo planar MR image of cat brain following reperfusionof an occluded middle cerebral artery.

FIG. 15 shows an echo planar MR image of cat brain with superimposedcontour map showing 20% or greater signal hyperintensity.

FIG. 16 shows an echo planar MR image of cat brain with superimposedcontour map of signal hyperintensity.

FIG. 17 shows a histopathologic section of cat brain after occlusion ofa middle cerebral artery.

FIG. 18 shows a histopathologic section of cat brain after occlusion ofa middle cerebral artery with superimposed contour map.

FIG. 19A shows a brain image providing information from autoradiography.

FIG. 19B shows the corresponding brain contour map providing informationfrom autoradiography.

FIG. 19C shows a brain image with superimposed contour map providinginformation from autoradiography.

FIG. 20A shows a histopathologic brain section.

FIG. 20B shows the corresponding brain contour map providing informationfrom histopathology.

FIG. 20C shows a histopathologic brain section with superimposed contourmap.

FIG. 21A shows a T2-weighted MR image of cat brain without contrastagent.

FIG. 21B shows the corresponding brain contour map of MR signalintensity.

FIG. 21C shows a T2-weighted MR image with superimposed contour map.

FIG. 22 shows a T2-weighted MR image with contrast agent obtainedaccording to one practice of the invention.

FIG. 23A shows a T2-weighted MR image during unilateral MCA occlusion.

FIG. 23B shows the contour map of hyperintensity for the recorded imageof 23A.

FIG. 23C shows the superimposed results of combining 23A and 23B.

FIG. 24A shows a recorded MR image during unilateral MCA occlusion.

FIG. 24B shows the contour map of hyperintensity for the recorded imageof 23A.

FIG. 24C shows the superimposed results of combining 24A and 24B.

Thus the method of the invention provides a quantitative and temporaldetermination of local perfusion variations, e.g. deficits or increases,which may arise from, for example, stroke, microsurgery oradministration of blood flow modifying pharmaceuticals.

The method of the present invention is preferably carried out usingspin-echo techniques. Alternatively and also preferably the method maybe carried out using a so-called fast or ultra fast imaging technique inorder to enable a series of T₂* dependent images to be generated with asshort as possible a time interval between successive images. For thisreason, techniques capable of generating images with time intervals ofless than 5 seconds, especially less than 0.5 seconds and moreespecially less than 100 milliseconds, are particularly preferred. Thus,in general, techniques such as spin echo, gradient echo, TurboFLASH, andmost especially the various varieties of echo planar imaging (EPI), areparticularly suitable for use in accordance with the method of theinvention.

In the method of the invention, an indication of the degree of bloodflow abnormality or modification for a given voxel may readily bedetermined by comparison of the MR signal intensity for that voxel witha reference value, e.g. the signal intensity for similar tissue withnormal blood flow. The reference intensity values may be predeterminedor may be selected as the MR signal intensity values for voxels ofnormal tissue in the same image. In the case of cerebral ischaemias,signal intensity values from the normally perfused gray matter and whitematter of the brain may be used to provide reference values for theaffected tissue. As is discussed in further detail below, the locationand spatial extent of the blood flow abnormalities, and the location andspatial extent of the regions having the most severe blood flowabnormalities detected in this way according to the method of theinvention, correspond closely to the same extents and locations asdetermined using conventional non-MRI techniques such as histopathologictissue-staining and quantitative autoradiography.

In one particularly preferred embodiment of the invention, temporallyspaced images are generated following repeated administrations of the MScontrast agent, e.g. at intervals of no less than 15–30 minutes, wherebyto detect the time of onset and thereafter to monitor the development ofthe blood flow abnormality or modification, e.g. to identify the extentand location of reperfusable tissue and the degree of success ofreperfusion, or to identify tissue for which surgical intervention isrequired before reperfusion is possible.

Thus the method of the invention may be used to characterizequantitatively the regional microcirculation of the brain before andafter acute arterial occlusion and differentiate between regions withnormal blood flow, reduced blood flow and no blood flow. With thedifferent distributions of MS contrast media in occluded and reperfusedcerebral tissues, the method may also be used to document reperfusion ofischemic tissue. Moreover, with the use of MS contrast media in themethod of the invention, distinction can be made between central coresof necrosis and the surrounding penumbrae of salvageable tissue, i.e.between irreversibly and reversibly injured brain tissue. Usingultrafast imaging techniques in the method of the present invention, thekinetics of the distribution of the contrast medium into tissue and ofthe wash-out of the contrast medium from the tissue can be followed soas to provide a diagnostic “signature” which could be used todistinguish between normal, ischemic, infarcted and reperfused tissueand to characterize the type of ischemic event and to identify tissuesat risk from ischemia.

In one embodiment of the method of the invention, using a fast imagingprocedure, the determination of the location and severity of ischaemiais effected by determining the time dependence of the MR signalintensity for the voxels in the seconds following administration of thecontrast agent, and generating an image where voxel image intensityvalue is dependent on the time post-administration at which MR signalintensity for that voxel is lowest. Normal tissue reaches minimum MRsignal intensity sooner than ischaemic tissue, and the resulting imagethus enables the spatial extent and local severity of blood flowabnormality to be visualized. Alternatively, a similar image may begenerated by making the voxel image intensity value dependent on thetime taken before voxel MR signal intensity reattains a pre-selectedcontrol value, e.g. its pre-injection value or a percentage of thatvalue (for example 80%).

The contrast agent used according to the method of the invention shouldbe an intravascular contrast agent, that is to say one which issubstantially retained within the systemic vasculature at least until ithas passed through the body region or organ of particular interest.Generally, therefore, blood pooling, particulate and hydrophiliccontrast agents or contrast agents possessing more than one of theseproperties are of particular interest.

Besides its obvious application in terms of identifying and giving anindication of the severity of cerebral or cardiac ischemias or infarcts,the method of the present invention has a broad range of possiblediagnostic and evaluative applications of which the following list namesbut a few:

-   Assessment of cerebral perfusion in brain dysfunction associated    with acute severe symptomatic hyponatremia;-   Evaluation of new therapies (for example thrombolytic therapies and    clot removal, calcium channel blockers, anti-inflammatory agents,    angioplasty, etc) in the treatment of cerebral vasospasm;-   Assessment of cerebral perfusion following induced subarachnoid    haemorrhage;-   Assessment of different degrees of ischemia in large tissue masses;-   Study of the relationship between blood ammonia, lactate, pH and    cerebral perfusion in cerebral ischemia associated with acute liver    failure (this has implications for the treatment of Alzheimer's    disease);-   Localisation and assessment of thrombus and plaque; Evaluation of    new therapies for stroke (for example t-PA, aspirin    antiphospholipids/lupus anticoagulants, antiphospholipid antibodies,    etc);-   Evaluation of risk factors for stroke (for example elevated serum    lipids, etc);-   Assessment of the impact of induced brain hypothermia on cerebral    perfusion during neurosurgery for stroke;-   Assessment of the effects of ageing on cerebral perfusion including    the study of the etiology of lacunar infarcts;-   Assessment of the effects of cocaine, amphetamine and ethanol on    cerebral perfusion in mildly and severely ischemic brain;-   Definition of the “therapeutic window” in reversible focal ischemia    for heparin, vasodilators, antihypertensives and calcium    antagonists; and-   Monitoring of other induced vasodilator effects.

Thus viewed from a further aspect the invention provides a method ofdetecting and quantitatively evaluating the severity of ischemias in ahuman or non-human, especially mammalian, body, said method comprisingadministering into the systemic vasculature of said body a contrastenhancing amount of an intravascular paramagnetic metal containingmagnetic susceptibility magnetic resonance imaging contrast agent,subjecting said body to a magnetic resonance imaging procedure capableof generating from magnetic resonance signals from said body a series oftemporally spaced images of at least a part of said body into which saidagent passes, and detecting temporal variations in said signals orimages whereby to detect ischemic tissue and to provide a quantitativeindication of the degree of blood perfusion deficit therein.

Viewed from a still further aspect, the present invention also providesa method of monitoring the vasodilatory or vasocontractory effects of aphysiologically active substance administered to a human or non-humananimal body, for example a calcium antagonist, said method comprisingadministering said substance into said body, administering into thesystem vasculature of said body a contrast enhancing amount of anintravascular paramagnetic metal containing magnetic susceptibilitymagnetic resonance imaging contrast agent, subjecting said body to amagnetic resonance imaging procedure capable of generating from magneticresonance signals from said body a series of temporally spaced images ofat least a part of said body into which said agent passes, and detectingtemporal variations in said signals or images whereby to monitor thevasoconstriction or vasodilation induced by said substance.

Viewed from a still further aspect, the present invention also providesa method of monitoring surgically induced blood perfusion variations,either before or during surgery, said method comprising administering acontrast enhancing amount of an intravascular paramagnetic metalcontaining magnetic susceptibility magnetic resonance imaging contrastagent into the systemic vasculature of a human or animal body which isundergoing or has undergone surgery, in particular microsurgery on saidvasculature, subjecting said body to a magnetic resonance imagingprocedure capable of generating from magnetic resonance signals fromsaid body a series of temporally spaced images of at least a part ofsaid body into which said agent passes, and detecting temporalvariations in said signals or images whereby to identify regions ofsurgically induced variations in blood perfusion.

Viewed from a still further aspect the invention provides the use of aMS contrast agent for the manufacture of a contrast medium for use inthe methods of the invention.

The magnetic susceptibility, T₂*-reducing effect, of MS contrast agentsis to a large degree dependent on the magnitude of the magnetic momentof the magnetic species within the contrast agent—the higher themagnetic moment the stronger the effect. Indeed the effect isapproximately proportional to the square of the magnetic moment makingthe effect of Dy(III) about 1.95 times larger than that of Gd(III). Ingeneral paramagnetic metal species having magnetic moments of ≧4 BM willbe preferred. The contrast agents particularly preferred for use in themethod of the present invention are those containing paramagneticlanthanide ions, especially high spin lanthanides such as ions of Dy,Gd, Eu, Yb and Ho, in particular Dy(III).

In order that they may be administered at effective but non-toxic doses,such paramagnetic metals will generally be administered in the form ofionic or much more preferably non-ionic, complexes, especially chelatecomplexes optionally bound to larger carrier molecules which may beselected to manifest greater residence times in plasma, or to enhancethe blood pooling nature of the contrast agent or to reduce theosmolality of the contrast medium by increasing the number ofparamagnetic centres per contrast agent molecule (or molecular ion).

A wide range of suitable chelants, polychelants, and macromolecule boundchelants for paramagnetic metal ions has been proposed in the patentliterature over the last decade and in this respect particular regardmay be had to U.S. Pat. No. 4,647,447 (Gries), U.S. Pat. No. 4,687,659(Quay), U.S. Pat. No. 4,639,365 (Sherry), EP-A-186947 (Nycomed),EP-A-299795 (Nycomed), WO-A-89/06979 (Nycomed), EP-A-331616 (Schering),EP-A-292689 (Squibb), EP-A-232751 (Squibb), EP-A-230893 (Bracco),EP-A-255471 (Schering), EP-A-277088 (Schering), EP-A-287465 (Guerbet),WO-A-85/05554 (Amersham) and the documents referred to therein, thedisclosures of all of which are incorporated herein by reference.

Particularly suitable chelants for the formation of paramagnetic metalchelate MS contrast agents for use in the method of the presentinvention include the following:N,N,N′,N″,N″-diethylenetriaminepentaacetic acid (DTPA),6-carboxymethyl-3,9-bis(methylcarbamoyl-methyl)-3,6,9-triazaundecanedioicacid (DTPA-BMA),6-carboxymethyl-3,9-bis(morpholinocarbonylmethyl)-3,6,9-triazaundecanedioicacid (DTPA-BMO), 1,4,7,10-tetraazacyclododecane-N,N′,N″,N′″-tetraaceticacid (DOTA), 1,4,7,10-tetraazacyclododecane-N,N′,N″-triacetic acid(DO3A),1-(2-hydroxypropyl)-1,4,7,10-tetraazacyclododecane-N,N′,N″,N″-triaceticacid (HP-DO3A), 1-oxa-4,7,10-triazacyclododecane-N,N′,N″-triacetic acid(OTTA), polylysine-bound DTPA and DTPA derivatives or DO3A and DO3Aderivatives or DOTA and DOTA derivatives (eg. DTPA-polylysine,DO3A-polylysine and DOTA-polysine), soluble dextran-bound DTPA and DTPAderivatives having with a total molecular weight≧40 KD, preferably inthe range 60–100 KD (DTPA-dextran).

Particularly suitable paramagnetic metal ions for chelation by suchchelates are ions of metals of atomic numbers 21 to 29,42,44 and 57 to71, especially 57 to 71, more especially Cr, V, Mn, Fe, Co, Pr, Nd, Eu,Gd, Tb, Dy, Ho, Er, Tm, Yb and Ln, in particular Cr(III), Cr(II), V(II),Mn(III), Mn(II), Fe(III), Fe(II) and Co(II) and especially Gd(III),Tb(III), Dy(III), Ho(III), Er(III), Tm(III) and Yb(III) especiallyDy(III), Ho(III) and Er(III).

All paramagnetic ions have both T₁ and T₂ reducing effects on thesurrounding non-zero spin nuclei and as the effect on MR signalintensity of these two effects is generally opposed in unweightedimages, T₁ reduction leads to image intensity increases whereas T₂reduction leads to image intensity losses. Thus for the purposes of thepresent invention it is particularly preferred to use paramagneticmetals which have relatively poor T₁-relaxivity in order to maximize theMR effect of the contrast agents in T₂* or T₂ weighted MR imaging. ThusDy(III) or even Yb(III) would generally be used in preference toGd(III).

In order to perform the method of the invention with as high as possiblea safety factor, the ratio between the dose of the contrast agent andits LD₅₀, it is particularly preferred to use non-ionic or lowosmolality chelates, i.e. chelates which carry no overall ionic charge,such as Dy DTPA-BMA for example, or where the complex has an overallionic charge to paramagnetic metal centre ratio of 1.5 or less.

Furthermore, to ensure that the contrast agent remains wholly oressentially within the blood vessels during passage through the bodyregion of interest, the contrast agent will as mentioned abovepreferably be hydrophilic and retained in the vasculature for asufficiently long time to permit effective imaging.

Examples of suitable blood-pooling agents include the inert solublemacromolecule-bound chelates of the type described by Nycomed inEP-A-186947 and WO-A-89/06979. Binding the chelant to a macromolecule,e.g. a polysaccharide such as dextran or derivatives thereof, to producea soluble macromolecular chelant having a molecular weight above thekidney threshold, about 40 KD, ensures relatively long term retention ofthe contrast agent within the systemic vasculature.

Examples of suitable hydrophilic contrast agents include linear,branched or macrocyclic polyamino-carboxylic acid chelates ofparamagnetic metal ions, especially chelates of chelants in whichcarboxylic acid groupings are replaced by hydrophilic derivativesthereof, such as amides, esters or hydroxamates, or in which the chelantbackbone is substituted by hydrophilic groupings such as for examplehydroxyalkyl or alkoxyalkyl groups. Chelants of this type are disclosedfor example in U.S. Pat. No. 4,687,658 (Quay), U.S. Pat. No. 4,687,659(Quay), EP-A-299795 (Nycomed) and EP-A-130934 (Schering).

Particular mention however must be made of the Dy(III), Ho(III) andEr(III) chelates of DTPA-BMA, DTPA-BMO, and DO3A and HP-DO3A.

The dosages of the contrast agent used according to the method of thepresent invention will vary according to the precise nature of thecontrast agent used. Preferably however the dosage should be kept as lowas is consistent with still achieving an image intensity reduction inT₂*-weighted imaging. Thus for Dy(III) based chelates, for example,dosages of Dy of 0.05 to 0.5 mmol/kg bodyweight, and especially 0.08 to0.3 mmol/kg, are particularly preferred. In this way not only aretoxicity-related problems minimized but the sensitivity of the imagingmethod towards the detection of ischemia of varying degrees of severityis increased. At higher dosages the signal suppression by the MScontrast agent may be unduly abrupt and intense, making regions withrelatively minor perfusion deficits appear to have the characteristicsof relatively normal blood flow. For most MS contrast agents theappropriate dosage will generally lie in the range 0.02 to 3 mmolparamagnetic metal/kg bodyweight, especially 0.05 to 1.5 mmol/kg,particularly 0.08 to 0.5, more especially 0.1 to 0.4 mmol/kg. It is wellwithin the skill of the average practitioner in this field to determinethe optimum dosage for any particular MS contrast agent by relativelyroutine experiment, either in vivo or in vitro.

Where the contrast agent is ionic, such as is the case with Dy DTPA, itwill conveniently be used in the form of a salt with a physiologicallyacceptable counterion, for example an ammonium, substituted ammonium,alkali metal or alkaline earth metal cation or an anion deriving from aninorganic or organic acid. In this regard, meglumine salts areparticularly preferred.

Contrast agents may be formulated with conventional pharmaceutical orveterinary aids, for example stabilizers, antioxidants, osmolalityadjusting agents, buffers, pH adjusting agents, etc., and may be in aform suitable for injection or infusion directly or after dispersion inor dilution with a physiologically acceptable carrier medium, e.g. waterfor injections. Thus the contrast agents may be formulated inconventional administration forms such as powders, solutions,suspensions, dispersions etc., however solutions, suspensions anddispersions in physiologically acceptable carrier media will generallybe preferred.

The contrast agents may therefore be formulated for administration usingphysiologically acceptable carriers or excipients in a manner fullywithin the skill of the art. For example, the compounds, optionally withthe addition of pharmaceutically acceptable excipients, may be suspendedor dissolved in an aqueous medium, with the resulting solution orsuspension then being sterilized. Suitable additives include, forexample, physiologically biocompatible buffers (as for example DTPA orDTPA-bisamide (e.g.6-carboxymethyl-3,9-bis(methylcarbamoylmethyl)-3,6,9-triazaundecanedioicacid)) or calcium chelate complexes (as for example calcium DTPA salts,calcium DTPA-bisamide salts or NaCaDTPA-bisamide) or, optionally,additions (e.g. 1 to 50 mole percent) of calcium or sodium salts (forexample, calcium chloride, calcium ascorbate, calcium gluconate orcalcium lactate and the like).

Parenterally administrable forms, e.g. intravenous solutions, should ofcourse be sterile and free from physiologically unacceptable agents, andshould have low osmolality to minimize irritation or other adverseeffects upon administration and thus the contrast medium shouldpreferably be isotonic or slightly hypertonic. Suitable vehicles includeaqueous vehicles customarily used for administering parenteral solutionssuch as Sodium Chloride Injection, Ringer's Injection, DextroseInjection, Dextrose and Sodium Chloride Injection, Lactated Ringer'sInjection and other solutions such as are described in Remington'sPharmaceutical Sciences, 15th ed., Easton: Mack Publishing Co., pp.1405–1412 and 1461–1487 (1975) and The National Formulary XIV, 14th ed.Washington: American Pharmaceutical Association (1975). The solutionscan contain preservatives, antimicrobial agents, buffers andantioxidants conventionally used for parenteral solutions, excipientsand other additives which are compatible with the contrast agents andwhich will not interfere with the manufacture, storage or use ofproducts.

In the method of the present invention where the lanthanide has anysignificant T₁-reducing effect, which is especially the case where theparamagnetic metal is Gd rather than Dy, this T₁-reducing effect mayalso be utilised to increase the degree of certainty with which perfusedregions are identified by generating corresponding T₁ weighted imagesand determining the signal ratio for each pixel or voxel between the twotypes of image. In this way tissue with very limited perfusion mayperhaps be distinguished from tissue in which blood flow has ceasedentirely. Where such a technique is used however it will be especiallydesirable to use the low toxicity, low osmolar forms of the paramagneticcomplex in order to operate with as large a safety factor as possible.Thus for Gd it will generally be preferably to use GdHP-DO3A, GdDO3A,GdDTPA-BMA or GdDTPA-BMO rather than GdDOTA or GdDTPA salts.

Generally data manipulation forms a major part of the method of theinvention since information regarding the severity of perfusion deficitmay be extracted from the rate at which signal intensity loss takesplace for the region of interest following MS contrast agentadministration (the less obstructed the blood flow the more rapidly thesignal is lost) and the duration and magnitude of signal loss. Clearlycomparison with data obtained for healthy tissue will enable a form ofperfusion calibration to be made. Moreover indications of the bloodvolume affected may also be obtained by measurement of the area underthe curve for a plot of pixel or voxel signal intensity loss over timefor the duration of the MS contrast agent induced signal loss. Thenecessary data manipulation, including display of zones of reduced orenhanced perfusion optionally superimposed on a selected backgroundimage, e.g. the “native” image obtained in the absence of the MScontrast agent, can of course be performed by a computer, generally thesame computer as is arranged to operate the MR imager and generate MRimages from the detected MR signals.

The methods of the invention are particularly suited to the earlydetection of ischaemias as ischaemic events may in this way be detectedsignificantly less than 1 hour after occurrence, as opposed to the 2–3hours or more of conventional T₂ weighted MRI, so making it possible totake steps to reperfuse the affected tissue at an earlier stage or totreat it with a cerebroprotective pharmaceutical, and thus raising thechances of reducing permanent tissue damage and of increasing tissuesurvivability.

The method of the invention will now be described further by way ofexample with particular reference to certain non-limiting embodimentsand to the accompanying drawings in which FIGS. 1 to 24 are images ordiagrams of the cat brain before, during or after unilateral MCAocclusion.

Study 1

Young adult cats weighing 2.0 to 4.5 kg were anaesthetized with 30 mg/kgi.v. Nembutal. Polyethylene catheters were placed in the femoral arteryand vein for blood pressure monitoring and drug administration. Theright middle cerebral artery (MCA) was isolated via the transorbitalapproach and occluded just proximal to the origin of the lateral striatearteries with bipolar electrocautery followed by complete surgicaltransection. The dural incision and orbit were covered with salinemoistened gauze and absorbable gelatin sponge.

A General Electric CSI (2 Tesla) unit, equipped with Acustar S-150self-shielded gradient coils (+20 gauss/cm, 15 cm bore size) was used.MRI was performed with an 8.5 cm inner-diameter low-pass birdcage protonimaging coil. Successive multislice T₂-weighted coronal images wereobtained for up to 12 hours following occlusion. Spin-echo T₂-weightedimages (TR 2800, TE 80 and 160, 3 mm slices, 1 mm gap) were obtainedwith a field-of-view (FOV) of 80 mm in which two scans were averaged foreach one of the 128 phase-encoding steps resulting in a totalacquisition time of 12 minutes.

In order to evaluate the anatomic region of perfusion deficiencyfollowing MCA occlusion, cats were injected with a non-ionicT2*-shortening contrast agent, DyDTPA-BMA. The DyDTPA-BMA complex wasprepared by refluxing an aqueous suspension containing stoichiometricamounts of dysprosium oxide and DTPA-BMA. The contrast agent was infusedi.v. at doses of 0.25, 0.5 or 1.0 mmol/kg beginning at phase-encodingstep #32 and finishing at step #60 (approximately 3 min) of T₂-weightedimage acquisition. DyDTPA-BMA injections were given at different timepoints post MCA occlusion in individual cats. After injection, themagnetic susceptibility effect was quantified for up to 60 minutes inboth ischemic and normal hemispheres by comparing region-of-interest(ROI) intensity to pre-contrast T₂-weighted ROI intensities. ROI imageanalyses were carried out in the ischemic inferior parietal gyrus,caudate, putamen, and internal capsule, and compared with thecorresponding uninjured contralateral regions. A signal intensity ratiowas calculated as the ROI image intensity ratio of an abnormal, ischemicregion over that of the normal, contralateral side. Results wereexpressed as the mean percentage change ±Standard Error of the Mean(X±S.E.M.)

At the conclusion of the MR protocol, 15 ml/kg of a 2% solution of2,3,5-triphenyl tetrazolium chloride (TTC) was infused transcardially.The brain was removed from the cranium after 10–20 minutes, immersed ina 2% TTC solution for another 10–20 minutes, and then stored overnightin 10% buffered formalin in a light shielded container. The brain wassectioned coronally (2–3 mm slices) from 24–36 hours later andimmediately examined for histologic evidence of ischemic damage asevidenced by pallor of TTC-staining.

Using a 0.5 mmol/kg dosage of DyDTPA-BMA, maximum signal intensitylosses of 35% were observed in the gray matter of the normalnon-occluded cerebral hemisphere during the first 15 minutes afterinjection. Signal intensity changes in white matter (internal capsule)in both the normal and ischemic hemispheres were smaller than in graymatter, presumably because of higher cerebral blood flow to gray matter.The resulting contrast-enhanced images had superior gray/white mattercontrast than T₂-weighted spin-echo MR images without contrast. At 45minutes after administration of DyDTPA-BMA, signal intensity hadrecovered to at least 90% of pre-contrast control values in all cerebraltissues. Increasing the dosage of DyDTPA-BMA from 0.5 to 1.0 mmol/kgproduced only a minimal difference in immediate post-contrast signalintensity. Long TE times (160 msec) produced the highest gray/whitematter contrast after DyDTPA-BMA at each of the 3 doses tested. (Ingeneral in the method of the invention using higher TE values leads to aslight loss in signal to noise ratio but also to increased sensitivityto T₂*—induced proton dephasing and hence to the MS contrast agent).

Perfusion deficits resulting from occlusion of the MCA were detected asregions of signal hyperintensity of the occluded ischemic tissuecompared to the normally perfused areas in the contralateral hemisphere.Relative hyperintensity was found in the occluded basal ganglia as earlyas 30 minutes post-occlusion for both the 1 mmol/kg and 0.5 mmol/kgdosages. Signal differences between ischemic and contralateral controltissues were observed for gray matter in the inferior parietal gyrus(42±14%), and basal ganglia (26±8%), and to a lesser extent, for thewhite matter in the internal capsule (5±4%). By comparison, T₂-weightedMRI without contrast failed to demonstrate any significant signaldifferences prior to approximately 2–3 hours post MCA occlusion (seeTable 1). As well, DyDTPA-BMA administration allowed detection of smalldeveloping infarcts that were not visible or were ambiguous onT₂-weighted images without contrast (see Table 2).

TABLE 1 Effect of DyDTPA-BMA administration on the time of detection ofcerebral ischemic damage. Onset of signal hypertensity Dose (relative topre-contrast DyDTPA-BMA T₂ - weighted image) (mmol/kg) # Cats TestedEarlier Same Time Later 0.25 5 1 4 0 0.50 16 12 4 0 1.0 6 4 2 0

TABLE 2 Effect of DyDTPA-BMA administration on the definition of injurysite (signal intensity ratio of injured tissue to correspondingcontralateral control tissue) compared to pre-contrast T₂ - weightedimage. Signal intensity ratio Dose (relative to pre-contrast DyDTPA-BMA# injections T₂ weighted image) (mmol/kg) contrast Better Same Worse0.25 5 4 1 0 0.50 23 17 6 0 1.0 16 14 2 0

Within 3–5 hours after MCA occlusion, T₂-weighted images alsodemonstrated tissue injury clearly, including increased mass-effect andhyperintensity (edema) throughout the MCA territory. The distribution ofincreased signal intensity correlated well anatomically with regions ofperfusion deficiency demonstrated with DyDTPA-BMA-enhanced MR imaging. Acontinuing close anatomic correspondence between areas of perfusiondeficit and edematous regions was seen 9 hours and 11 hours postocclusion. In subsequent TTC-stained coronal sections, these areas werefound to exhibit characteristics typical of ischemic tissue injury, suchas pallor of staining, coagulation, necrosis, and glial proliferation.

These results confirm that MS contrast agent enhanced MRI cansignificantly advance the time of detection of cerebral ischemicinsults. Evidence of stroke-induced perfusion deficits was observed inthe MCA territory as early as 45 minutes post-occlusion usingcontrast-enhanced MRI, whereas T₂-weighted spin-echo images withoutcontrast did not demonstrate increased signal intensity until 2–3 hoursafter occlusion.

Contrast in T₂-weighted spin-echo MRI can be produced by changes in themicroscopic magnetic fields experienced by protons undergoing moleculardiffusion. These field gradients cause spin dephasing and loss of spinecho signal intensity. Field gradients arise at the interface of twovolumes with different magnetic susceptibilities and thus differentinduced magnetic fields.

The presence of paramagnetic chelates can alter the magneticsusceptibility of tissue. In the brain, since the chelates are confinedto the intravascular space by the blood-brain barrier, a field gradientis induced between the capillary space and surrounding (perfused) tissueresulting in significant signal loss. These results show that thisapproach to MR contrast enhancement can be used to differentiateischemic from normally perfused regions.

A further notable advantage of the method of the invention is itsrelative insensitivity to motion compared to diffusion-weighted MRimaging. Given the relatively high safety index (LD₅₀ i.v.administration in mice is 34 mmol/kg), the long duration of the magneticsusceptibility effect of DyDTPA-BMA and its negligible T₁-reducingeffect makes this a particularly good MS contrast agent for use withT₂-weighted MRI.

The contrast-enhanced images suggested considerable regionalheterogeneity in perfusion throughout the ischemic MCA territory.Post-contrast signal hyperintensity was observed earlier in the basalganglia than the neocortex. This finding suggests that non-anastomosingend-arterial tissues, such as the caudate and putamen, are mostsusceptible to post-ischemia perfusion deficits, since no collateralcirculation is available. In collaterally perfused areas such asneocortex, on the other hand, tissue injury may be mitigated somewhat bycontinued blood flow in the partially ischemic watershed regions. Itseems likely that the method of the invention may be able to helpidentify normal and abnormal regional blood flow differences, andespecially to differentiate reversibly ischemic penumbra from infarctedtissue based on the degree and duration of perfusion deficit to cerebraltissues.

In further studies of cerebral perfusion deficits on the cat MCA modeldescribed above, using echo planar imaging in conjunction with low, ie.0.1 and 0.15 mmol/kg, dosages of DyDTPA-BMA, quantitative spatial andtemporal assessment of stroke affected tissue may be made. This dosagereduction further increases both the potential sensitivity and thesafety profile of the method of the invention.

The advantage of using echo planar MRI with a MS contrast agent is thatimages may be acquired continuously before, during and after contrastinjection. This allows the time course of the contrast agent passagethrough a tissue to be monitored and to obtain images at the maximumcontrast dosage. Since the echo-planar pulse sequence does not require a180° refocussing pulse, the T₂* effects of the MS contrast agent areaccentuated.

Echo planar images on the GE CSI 2 Tesla were acquired in a sequentialfashion. Sixteen images were obtained one each second or less, eachimage possessing a 66 msec acquisition time with a data matrix of 64×64pixels over a 60×60 mm field-of-view. The slice thickness was 3 mm. Theecho-planar sequence was that of a gradient-echo nature, with the timeof echo (TE) value adjusted to maximise the T₂*-shortening contrasteffect.

Study 2

The method of the invention is further illustrated by the images shownin FIGS. 1 to 11.

T₂-weighted spin-echo (TR/TE 2800/180 msec) images of the cat brain wereacquired following a unilateral MCA occlusion of one hour duration andsubsequent reperfusion of the occluded artery.

At 108, 160 and 320 minutes following arterial reopening 0.5 mmol/kg DyDTPA-BMA was injected intravenously over 90 seconds between phaseencoding-step steps 32 and 60 of 128 phase encoding image acquisitions.

A section of the unaffected brain hemisphere was selected as a signalintensity reference (100%) and for each temporal image of the selectedslice contours showing the degree of the hyperintensity of the affectedhemisphere were plotted. The images at 108, 160 and 320 minutespost-occlusion are shown in FIGS. 1, 2 and 3. The corresponding contourmaps of hyperintensity, i.e. blood perfusion deficit, are shown in FIGS.4, 5 and 6 and, superimposed on the MR images, in FIGS. 7, 8 and 9.

The accuracy of this technique in identifying the location, extent andseverity of ischaemic damage is demonstrated by the correspondingTTC-stained histopathologic sections shown in FIG. 10 and, withsuperimposed staining contours, in FIG. 11.

These images illustrate a failed reperfusion which led ultimately tosevere and extensive brain damage, which was corroborated by thehistopathologic results.

Study 3

The method of the invention is further illustrated by the images shownin FIGS. 12 to 18.

Echo planar (EP) images (65 msec acquisition time) of the cat brain wererecorded before, during and after a unilateral MCA occlusion, in eachcase with intravenous bolus injection of 0.25 mmol/kg DyDTPA-BMA betweenthe first and second of sixteen sequential EP images.

A region of the unaffected brain hemisphere was selected as a signalintensity reference (100%) and for each temporal image of the selectedslice the extent and the severity of perfusion deficit was plotted. Theimages before occlusion, during occlusion and after successfulreperfusion are shown in FIGS. 12, 13 and 14. The area showing 20% orgreater hyperintensity, indicative of the extent of the perfusiondeficit, is shown in FIG. 15 and a contour map of hyperintensityillustrating the areas of severe deficit is shown in FIG. 16, in eachcase for the image acquired during occlusion as the pre- andpost-occlusion images did not show hyperintense regions. Once again thearea and severity of blood flow reduction during occlusion thusidentified closely correlates with the corresponding TTC-stainedhistopathologic sections as shown in FIGS. 17 and 18.

Study 4

The method of the invention is further illustrated by the images shownin FIGS. 19 to 22.

The information on extent and severity of perfusion deficit availableusing the method of the invention was compared with that available usingconventional techniques of autoradiography (using 99Tc-HMPAO) and TTChistopathology. FIGS. 19 a, b and c, 20 a, b and c, 21 a, b and c and 22a, b and c show respectively a 99Tc-HMPAO autoradiograph, TTC-stainingof a histopathologic section, a T₂-weighted MR image (without contrastagent) and a T₂-weighted MR image following iv administration of 0.25mmol/kg DyDTPA/BMA. The (a) images are the images as recorded, the (b)images show contour maps of regions of modified intensity (the selectedreference (100%) regions are also shown) and the (c) images superimposethe raw images (a) and the contour maps (b). The correlation between theinformation from the autoradiograph (FIG. 19), which is currentlyaccepted as being particularly accurate and sensitive to cerebral bloodvolume determination and from the images obtained according to theinvention (FIG. 22) is particularly good.

Study 5

The method of the invention is further illustrated by the images shownin FIGS. 23 and 24.

T₂-weighted spin-echo (TR/TE 2800/180 msec) images of the cat brain wererecorded during unilateral MCA occlusion, more particularly at 128minutes and 280 minutes after arterial occlusion occurred. 0.25 mmol/kgDyDTPA/BMA was injected intravenously over 45 seconds between phaseencoding steps 32 and 60 of the 128 phase encoding-step acquisitions.FIGS. 23 a, b and c and 24 a, b and c show the recorded images at 128and 280 minutes (the (a) images), the contour maps of hypertensity (the(b) images showing the reference areas (100%) of the unaffectedhemisphere) and the superpositions of the MR images and the contour maps(the (c) images). At 128 minutes seven different regions of perfusiondeficiency were identified. This heterogenicity of the perfusiondeficiency is to be expected early in the course of a cerebralischaemia. At 280 minutes the increased levels of hyperintensity confirmworsening perfusion deficit in most brain regions but the heterogeneityof the hyperintensity suggests that some brain areas may still retainsome blood flow.

1. A method of detecting blood flow abnormality or variation in a vesselor tissue comprising: administering a contrast enhancing amount of aparamagnetic metal containing magnetic resonance magnetic resonancecontrast agent into a vessel of a body; imaging at least a portion ofthe body through which the MR contrast agent passes, with a MR imagingtechnique, thereby collecting temporally spaced sets of 3-D and 2-Ddata, each data set collected successively through an acquisition time;forming a time sequence of image data including early image data andlater image data; comparing 3-D and 2-D data from the temporally spacedsets of data by evaluating 2-D and 3-D temporally acquired images bycomparing ones of said early image data within said acquisition timewith ones of said later image data within said acquisition time andtheir intensity to assess blood flow or angiographic abnormality orvariation.
 2. A method of detecting blood flow abnormality or variation,in a human body, said method comprising the steps of: administering intovasculature of said human body a contrast enhancing amount of aparamagnetic metal containing magnetic resonance contrast agent;subjecting said human body to a magnetic resonance image procedurecapable of generating from magnetic resonance signals from said humanbody successive images of temporally spaced images taken over anacquisition time period of at least part of said human body into whichsaid contrast agent passes, said procedure being a magnetic resonanceimaging procedure; detecting temporal variations in said signals orimages; and from said temporal variations identifying regions ofabnormal or modified blood flow in said human body and providing aquantitative indication of blood flow abnormality or variation.
 3. Amethod of detecting and quantitatively evaluating the severity of bloodflow abnormality in a human body, said method comprising the steps of:administering into vasculature of said human body a contrast enhancingamount of a paramagnetic metal containing magnetic resonance contrastagent; subjecting said human body to a magnetic resonance imageprocedure capable of generating from magnetic resonance signals fromsaid human body successive images of temporally spaced images taken overan acquisition time period of at least part of said human body intowhich said contrast agent passes, said procedure being a magneticresonance imaging procedure, to detect temporal variations in saidmagnetic resonance signals or images; detecting blood flow abnormalityor flow variation in obstructed blood vessels in said body; andidentifying from said temporal variations in said images the blood flowabnormality.
 4. A method of detecting blood flow abnormality orvariation in a blood vessel comprising: administering a contrastenhancing amount of a paramagnetic metal containing magnetic resonancecontrast agent into a blood vessel of a body; imaging at least a portionof the body through which the MR contrast agent passes, with a magneticresonance imaging technique, thereby collecting temporally spaced setsof contour data and planar image data, each data set collectedsuccessively through an acquisition time; forming a time sequence ofimage data including early image data within said acquisition time andlater image data from within said acquisition time; comparing contourdata and planar image data from the temporally spaced sets of data byevaluating contour data and planar image data temporally acquired imagesby comparing ones of said early image data with ones of said later imagedata and their intensity to assess blood flow abnormality or variation.5. The method of claim 4 wherein said comparing step is carried out by aphysician visually examining at least two sequenced images.
 6. Themethod of claim 4 wherein said comparing step is carried out by softwarequantitatively manipulating contour data and planar image data from atleast two temporally spaced sets of data.
 7. The method of claim 2wherein the magnetic resonance imaging procedure is a fast, high speedor single shot imaging procedure.